The present invention relates to an aqueous powder coating dispersion which is particularly suitable as a coating for car bodies which have been coated with aqueous basecoat.
For the coating of car bodies, preference is currently given to the use of liquid coating materials. These cause numerous environmental problems owing to their solvent content. The same applies to cases where water-based coating materials are employed.
For this reason, increased efforts have been made in recent years to use powder coatings for the coating operation. The results so far, however, are not satisfactory; in particular, increased coat thicknesses are necessary in order to achieve a uniform appearance. On the other hand, the use of pulverulent coating materials entails a different application technology. Plants designed for liquid coating materials, therefore, cannot be used for the powders. The aim is therefore to develop powder coatings in the form of aqueous dispersions which can be processed with liquid coating technologies.
U.S. Pat. No. 4,268,542, for example, discloses a process in which a powder coating slurry is used which is suitable for the coating of cars. In this process, a conventional powder coat is first applied to the body, and the clearcoat slurry is applied as second coat. In this clearcoat slurry, which is based on acrylate resins, ionic thickeners are used. Furthermore, in one of the examples these thickeners contain from 0.5 to 30% of glycidyl-containing monomers. Moreover, high stoving temperatures (above 160.degree. C.) are necessary.
The present invention has now set itself the object of providing an aqueous powder coating dispersion which can be applied to car bodies by the existing liquid coating technology and which, in particular, can be stoved even at temperatures of 130.degree. C.